One type of conventional decompression device of an internal combustion engine is a centrifugal-weight type decompression device, such as is disclosed in Japanese Examined Patent Application Publication No. H02-25007. In this decompression device of centrifugal-weight type, as shown in FIG. 1, a plurality of centrifugal weights 103 and 104, which open outwardly in the radial direction of a cam shaft 101 by a centrifugal force, are swingably attached in an end portion of the cam shaft 101. The cam shaft 101 includes an exhaust-side cam face 110. One of the centrifugal weights 104 is provided with a decompression cam portion 105. Both the centrifugal weight 103 and 104 are biased in the radially inward direction by a spring (not shown).
When starting the engine, the decompression cam portion 105 pushes up an exhaust-side tappet 106 only by a predetermined distance away from a base face of the exhaust-side cam face 110 at a compression stage of the engine and, thus an exhaust valve (not shown) is opened to release/decompress a compression pressure inside a combustion chamber (not shown). When an engine speed reaches to a practical engine speed, the centrifugal weights 103 and 104 are swung and opened in the outward direction by the centrifugal force due to the rotation of the cam shaft 101 while resisting a biasing force of the spring. The decompression cam portion 105 then withdraws under the base face of the exhaust-side cam face 110 and, thus, a decompression action is terminated.
The following four conditions are known to occur in internal combustion engines having centrifugal-weight type decompression devices as discussed above. (1) Since an engine speed for operating the decompression (also referred to as “a decompression operating engine speed”) is determined in accordance with a balance of the centrifugal force and the biasing force of the spring, both of which act on the centrifugal weights 103 and 104 and, therefore the decompression operating engine speed is uniquely determined by the original setup of the decompression device, it is difficult to change the decompression operating engine speed afterwards. If the decompression operating engine speed is desired to be changed, for example, the decompression device must once be disassembled and then the centrifugal weights 103 and 104 must be replaced with those of different weight, or the spring must be replaced with that of a different spring force. Therefore, the replacement takes up a large amount of time and is really troublesome.
In many cases, an engine speed for terminating the decompression (also referred to as “a decompression terminating engine speed”) is typically set higher than an engine speed at which the engine is started by a starter motor and lower than an idling engine speed. However, when the idling engine speed is set close to an engine speed at the time of starting of the engine, the operating state of the decompression may continue during the idling after the starting of the engine, due to an existence of variations in the decompression operating engine speed caused by variations in an attachment position or a size of the centrifugal weights or a spring force when assembled.
Further, in the above centrifugal-weight type decompression device, the centrifugal weights 103 and 104 rotate in a vertical plane around the cam shaft 101, which is horizontally arranged. The centrifugal force varies while rotation with the influences of gravity as the centrifugal weights 103 and 104 pass lower and upper position along a rotating path thereof. Thus, it is difficult to set the decompression operating engine speed exactly at a desired value.
(3) Only one centrifugal-weight type decompression device is required for a single-cylinder internal combustion engine. However, if it is applied to an internal combustion engine in which each cylinder exists independently, such as a V-twin type internal combustion engine, centrifugal-weight type decompression devices must be provided to each cam shaft of each cylinder and, thus, the number of parts, weight, size, and so on of the internal combustion engine will increase.
(4) The centrifugal-weight type decompression device only allows a control based on the decompression operating engine speed which is preset when it is assembled, and it cannot perform a control depending on other engine speed conditions. That is, the device cannot function appropriately, for example, in the case where it is used at the time of a stop operation of an internal combustion engine and when a decompression is desired to be performed at an engine speed higher than the idling engine speed, or otherwise when a decompression is not desired to be performed until stop of the engine.